I've just a look and you need to download this file, unzip to get the portrait editor (not needed) and randomportraits.zip. Rename away the old version of randomportraits in your mod directory, if you have it. Unzip randomportraits.zip into the mod directory to create Strive/mods/randomportraits. Start Strive, select mods from the menu, then select randomportraits and Apply. Exit Strive, restart, and randomportraits should be creating images for new slaves and captures automatically.
Note - This does not include portrait packs. Look for those elsewhere on the forum.